This invention relates generally to VAR generators and it relates in particular to automatic non-linear frequency dependent dynamic gain adjusters for the control systems of VAR generators.
Static VAR generators compensate for the effects of highly reactive loads, such as arc furnaces, on the terminal variables of an AC supply system. The static VAR generator or flicker compensator as it is sometimes called, includes a control circuit which measures the magnitudes of the arc furnace currents in consecutive half cycles of the line or supply voltage. The measured information is used to compute necessary compensating currents in terms of required firing angle signals for the thyristors of the static VAR generator. The static VAR generator output can only be adjusted once during each half cycle per phase. Generally, prior art concerning static VAR generators is directed towards apparatus and method for determining the proper firing angle for the inductor which controls thyristors of the VAR generator. The trend in the prior art seems to be in a direction from apparatus associated with the determination of the firing angle by any means whatsoever towards determining the firing angle at a point in time as close as possible to the actual firing of the thyristors. Examples of that trend can be found in U.S. Pat. No. 3,936,727 issued Feb. 3, 1976 to F. W. Kelly, Jr. and G. R. E. Lezan which teaches a compensation control device which determines firing time for a static switch in accordance with the magnitude of the reactive load current and which additionally teaches a regulating means which controls the firing time to maintain the line current and line voltage (at a selected line location) in substantial phase coincidence. U.S. Pat. No. 3,999,117 issued Dec. 21, 1976 to Gyugyi et al teaches a static VAR generator and compensator where time delayed firing angles are calculated by integrating furnace load currents over predescribed intervals during real time to thus maintain balanced load current at specified phase angles--relative to phase voltage--which are usually zero for a three-phase electrical system. In a later patent, U.S. Pat. No. 4,000,455 issued Dec. 28, 1976 to Gyugyi, it is taught that the total computation time for determining the firing angle in each succeeding half cycle will take no longer than the length of time of the last complete half cycle (measured at voltage peak) immediately prior to the firing of the thyristor. In U.S. Pat. No. 4,068,150 by Gyugyi et al, it is determined that the computation time for determining the firing angle can actually extend into the half cycle in which correction is to be applied. In copending Application Ser. No. 880,270 also by Gyugyi et al, the computation time for determining the firing angle is extended even further into the half cycle in which correction is to be applied. It has been noted, however, that the accurate reliable timely determination of firing angle from circuit variables becomes less significant if the transport or delay time of the control system through which this information must be propagated--from initial sensing to application of thyristor firing control--is too large to allow the control system to respond to frequency of the load disturbance for which the VAR generator is provided to correct. The control system for the VAR generator must have a high gain in the frequency band where the VAR generator is capable of providing effective compensation against load disturbances. On the other hand, however, the gain of the control should be low at higher frequencies at which the VAR generator cannot follow the load variation, in order to eliminate the possibility that the VAR generator itself produces voltage variation. This can happen when the VAR generator, due to its own inherent time delay or transport time gets so far out of step that it provides reactive volt amps (VARS) which tend to reinforce rather than reduce the reactive variation of the load. It would be desirable, therefore, to provide a VAR generator having a control system where reduction of the control system gain is provided for high frequency disturbances of large magnitude.